1. Field of the Invention
The present invention relates to a fluid apparatus for controlling the operation amount (stroke) of the piston rod of a hydraulic cylinder.
2. Prior Art
In a known fluid apparatus of this kind, positioning of a piston rod, which is hollow, in a pneumatic cylinder is effected by converting a linear movement of the piston rod into the rotation of a ball screw shaft and detecting the amount of the rotation. In this fluid apparatus, a piston having the hollow piston rod is slidably mounted in a tube of the pneumatic cylinder, and a ball nut is fixed to the piston coaxially with this piston, and a ball screw shaft engaged with the ball nut and extending in the hollow piston rod is rotatably and axially unmovably mounted to a main body of the cylinder. A magnetic brake for braking the rotation of the ball screw shaft is mounted between the main body of the pneumatic cylinder and the ball screw shaft, and a rotary encoder which generates and outputs pulse signals in accordance with the angle of rotation of the ball screw shaft is mounted at an end of the ball screw shaft. A controller compares the pulse signal, received from the rotary encoder, indicating an actual position of the piston rod with a signal indicating a preset target position. When the levels of both signals coincide, the controller provides a braking signal to the magnetic brake and stops the supply of drive air to the piston so as to stop the piston rod at the target position. In this fluid apparatus, as the linear movement of the piston rod is converted into the rotation of the ball screw shaft through the ball nut, and stopping of the linear movement of the piston rod is effected by stopping the rotation of the ball screw shaft by means of the magnetic brake, this piston rod slips less and is stopped more reliably than a piston rod directly clamped (not through the ball nut). Further, advantageously, energy required for the braking operation is reduced. In addition, the piston rod can be held by a great force because the magnetic brake is used.
The above-described fluid apparatus has, however, the following problems. Firstly, because the magnetic brake keeps on acting on the ball screw shaft for a period of time corresponding to the speed of the piston rod, the magnetic brake is subject to abrasion on a friction surface thereof and the pressing force of the friction surface thereof varies with time. As a result, the braking force to be applied to the ball screw shaft becomes uneven, and accuracy in stopping or positioning the piston rod at a desired position or in controlling the speed of the piston rod decreases gradually. Secondly, magnetic brakes cause greater vibration and shock and reduce the speed of piston rods at a higher rate than other kinds of brakes. For this reason in addition to the continuous braking operation of the magnetic brake as described above, the magnetic brake and the main body of the pneumatic cylinder are not durable and the load-applied side of the main body adjacent to the piston rod is subject to the vibration generated by the magnetic brake.